FIELD OF THE INVENTION
The invention relates to a method for controlling the fuel supply for an internal combustion engine being equipped with a lambda regulating device, a catalytic converter disposed in an exhaust pipe of the internal combustion engine, a device for externally heating the catalytic converter to its operating temperature, and a secondary-air pump which injects additional air into the exhaust pipe.
It is known that the emission of harmful substances from an internal combustion engine can be reduced effectively by catalytic aftertreatment. In that case, a three-way catalytic converter in conjunction with a lambda regulating device has proved to be a high-performance concept for the aftertreatment of exhaust gas. The emission of harmful substances from an internal combustion engine is greatest in the cold-starting phase. The reason therefor is that the still cold catalytic converter can perform its function as a converter of harmful substances only when it has reached its so-called start-up temperature (light-off temperature) of around 300.degree. C. In the normal travel mode that takes, on average, one minute to two minutes, but appreciably longer in city traffic due to the low engine load and the consequently lower exhaust gas temperature. Various measure have already been disclosed for reducing that time, after which virtually complete conversion of the exhaust gas constituents HC, CO and NO.sub.x takes place.
Thus, it is known from German Published, Non-Prosecuted Patent Application DE 27 19 252 to heat the catalytic converter of an internal combustion engine electrically. However, since the catalytic converter has a considerable size and mass, it is necessary, for heating which is to take place in a short time, to have a very high heating power and therefore a high current demand which, when the internal combustion engine is used in an automobile, exceeds the capacity of the automobile current supply system.
A secondary-air system can be seen from the publication entitled: PIERBURG Produktinformation "Elektrische Sekundaerluftpumpe" [PIERBURG Production Information "Electric Secondary-air pump"], No. 5/400-151.01,9/91 published at the International Automobile Exhibition, Frankfurt/Main, 1991. That system, in an internal combustion engine, provides the injection of secondary air into the exhaust manifold during the cold, not yet operational phase of the catalytic converter.
That ensures that the harmful substances CO and HC, which occur to an increased extent in the cold phase as a result of an engine-side air coefficient of lambda &lt;1, are reduced and heat is supplied to the catalytic converter in order to achieve more rapid operational readiness after the cold start. The secondary-air injection system is formed of an air pump, one or more non-return valves and a cutoff valve.
It is known from German Published, Non-Prosecuted Patent Application DE 41 32 814 A1, in connection with such a secondary-air system, to heat the catalytic converter to operating temperature through the use of a burner which is operated with fuel of the internal combustion engine and reacts to control signals from a control unit. In that case, the heating power is at maximum immediately after the starting of the engine and can be controlled as a function of the temperature of the catalytic converter and/or of the lambda probe.